The polymerization of ethylene over a metallocene catalyst supported on amorphous fibrous silica nanoparticles of 250–300 nm in diameter has been investigated. The fibrous silica nanoparticles with large surface area and open pore entrances offer minimal diffusion resistances for the catalyst immobilization onto the silica surface. These fibrous silica particles are synthesized using reverse micelles as a template to grow silica emanating outward as protrusions or platelets from a small silica core. The fibrous silica platelets are free of internal pores and the metallocene catalyst components are anchored onto the external surface of each silica platelet. With the progress of polymerization, partial fragmentation of silica platelets occurs but the main body of the fibrous silica nanoparticles remains intact and they grow into polymer particles of 3–5 μm. Since all the active sites are present on the external surface of the silica platelets, the catalyst activity has been found to be much higher than the conventional microporous silica particles‐supported catalyst with comparable surface area.

Summary:In heterogeneously catalyzed polymerization of a-olefins, the characteristics of a solid support material impact the catalyst activity, polymer particle morphology, and resulting polymer properties. Silica is the most widely used support for metallocene catalysts in a-olefin polymerization processes because of its large surface area and favorable surface properties for catalyst anchoring. Understanding the kinetics of heterogeneous olefin polymerization over a solidsupported catalyst is often quite complicated because of mass transfer effects and catalyst particle fragmentation during the polymerization. Incomplete or premature fragmentation of support material results in a large fraction of catalyst sites left unavailable for the polymerization, causing some inconsistencies in the performance of the catalyst. Silica-supported metallocene catalysts for a-olefin polymerization are known to follow the layer-by-layer fragmentation mechanism where the fracture of the silica/polymer layer begins from the surface region of a silica particle and it gradually continues into the center of the particle as fragmentation is complete. In this paper, we present new experimental results on ethylene polymerization with rac-Et(indenyl) 2 ZrCl 2 /MAO catalyst using different types of silica supports to quantitatively assess the effects of support geometry on intrinsic catalytic activity. Flat surface silica, nano-sized spherical silica, straight cylindrical pore silica, and conventional silica are used as supports. The presence or absence of intraparticle monomer diffusion resistance and particle fragmentation has been shown to have significant effects on the catalytic activity.